The present invention is directed to an improved electrochemical oxidation process for forming quinonoid from corresponding aromatic and alkyl aromatic compounds in good yields and high selectivity. More specifically, the invention described and claimed herein requires the use of an aqueous, acidic solution containing ceric oxidant in the presence of a catalytic amount of chromium cations.
The quinones obtainable by the present process have a wide variety of known utility. For example, naphthoquinone, is a known additive in the paper making industry. Other quinones are useful as intermediates in forming fragrance components for perfumes as well as in forming pharmaceuticals. The products achieved by the present invention have been previously formed by a variety of processes which may be generally classified as chemical or electrochemical. Oxidation of aromatic compounds has been achieved by direct electrochemical oxidation in the presence of dilute acid electrolytic solutions as described in U.S. Pat. Nos. 4,298,438 and 4,354,904 as well as by indirect electrochemical oxidation in which the oxidant is electrolytically generated and, in turn, used to oxidize the aromatic compound.
Compounds which are known to be capable of acting as an indirect oxidant include the polyvalent metal salts, particularly the metals of cobalt, chromium, manganese, iron, lead, silver and cerium. Because regeneration of the spent metal to its higher oxidation state is not always highly effective and/or other insoluble salts are formed, those skilled in this art tend to use the salts of chromium, manganese, cobalt, iron or lead as these salts are less expensive and replacement of spent materials do not greatly detract from the economics of the process. However, each of these metal ion oxidants have certain properties which cause them to make the oxidation process ineffective. For example, chromium ions, when used as the sole oxidant, exhibits low reactivity and poor selectivity towards the desired products, cerium and manganese salts are believed to have low reactivity either as the oxidant or with respect to regeneration of the oxidant specie, the higher oxidation states of silver, cobalt and lead ions are not very stable and, in the case of iron, is not very reactive.
The ceric ion is a well known oxidizing agent in organic chemistry. It has the potential of presenting an excellent one electron oxidant but has not been previously used extensively or on an industrial scale because it has been associated with poor reactivity and selectivity. The cerium salts are prohibitively expensive and must, therefore, be capable of being stable, react with the organic substrate cleanly and be easily regenerated to its higher valence state. For an effective industrial process it is desirable to produce the product in a high yield based on substrate and the oxidant. This has been difficult to accomplish for some products, such as 2-methyl-1,4-naphthoquinone. West German patent No. 2952709 describes the production of 2-methyl-1,4-naphthoquinone from 2-methylnaphthalene using CrO.sub.3 in aqueous H.sub.2 SO.sub.4. While the yield based on substrate was 71%, the yield based on CrO.sub.3 was only 17%. Japanese patent No. 60/252445 shows the same oxidation using Ce(SO.sub.4).sub.2 in aqueous sulfuric acid as the oxidant. The yield was 24% based on substrate and 47% based on Ce(IV).
Canadian Patent No. 1,132,996 to Oehr describes a process for oxidizing naphthalene to naphthaquinone using ceric sulfate in dilute sulfuric acid. The low solubility limitations of cerium sulfate [Solubilities of Inorganic and Organic Compounds, Vol. 3, Part I, Ed. by H. L. Silcock (1974)]cause low efficiency and low yields as well as the need for large volumes of solution to oxidize small quantities of the organic compound.
It must be understood that although cerous/ceric ions have been known and used in oxidation reactions, there is a need to enhance the system to provide a commercially attractive process. The present process provides a catalyzed system which causes the cerium to be highly selective in forming the desired quinonoid compounds, to be capable of exhibiting high reaction rates and yield and to be capable of undergoing repeated cycling between its cerous (Ce.sup.+3) and ceric (Ce.sup.+4) species in a high degree of efficiency under the reaction and electrolysis conditions to make the process attractive on a commercial scale.